Field of the Invention
The present invention relates to an image signal coding apparatus, and particularly to a moving image coding apparatus and method that motion-compensation prediction-code a moving image signal including a plurality of color components having the same sample ratio, independently for each color component.
Description of the Related Art
Conventionally, a digital video camera has been well known as a camera-integrated moving image recording apparatus that takes an image of an object, compression-codes moving image data obtained by taking the image, and records the data.
MPEG2 capable of compression at a high compression rate using inter-frame motion prediction has typically been used as a compression coding scheme. In particular, H.264 achieving a high compression rate using a more complex prediction scheme has also been used recently. These compression coding schemes adopt a standardized input signal format, which is mainly called 4:2:0 format.
According to the 4:2:0 format, a color moving image signal, such as of RGB, is converted into a luminance component (Y) and two color difference components (Cb, Cr), and the number of samples of the color difference component is reduced to half of that of the luminance component both in vertical and horizontal directions.
The color difference component has lower visibility than the luminance component has. Accordingly, the amount of original information of a coding target is reduced by down-sampling of the color difference component before coding. Instead, the 4:2:2 format that down-samples, to the half, the color difference component only in the horizontal direction sometimes is used as a format for business use, such as a format of a broadcast video source.
Recently, according to highly-increasing resolution of degree of half-tone of a display, a coding scheme has been developed that codes the color difference component at the same number of samples as that of the luminance component without down-sampling the color difference component.
A format having the same number of samples of the color difference component as that of the luminance component is called the 4:4:4 format.
In coding of 4:2:0 format, information for motion-compensation prediction is multiplexed only on the luminance component, and motion compensation is performed on the color difference component using the information of the luminance component.
The coding of the 4:2:0 format is accomplished with the features that almost of image information is concentrated in the luminance component and distortion visibility is higher for the luminance component than for the color difference component so that the color difference component makes less contribution to image reproductivity.
In contrast, in coding of the 4:4:4 format, the three components have equivalent image information amount. Accordingly, a coding scheme different from the conventional coding scheme which assumes that the image information largely depends on one component as in the 4:2:0 format is desired.
A coding scheme in conformity with the 4:4:4 format is disclosed in, for instance, Japanese Patent Application Laid-Open No. 2007-133823. According to Japanese Patent Application Laid-Open No. 2007-133823, to the input image including three primary colors of an RGB signal, motion vector detection is performed on both the G component and luminance component (Y component) generated from the RGB component, and subsequently the correlation between each vector is calculated. If the correlation between the G component vector and the Y component vector is high, the motion vector of the Y component is applied to the motion vectors of the R, G and B components.
The coding process of the coding scheme in conformity with the 4:4:4 format performs motion prediction for each color component. Accordingly, no distinct difference in the inter-picture difference value sometimes appears between the reference image and the coding-target image on a certain color component.
As a result, unfortunately, coding is performed with the motion vector indicating the image position different from the actual motion of the object being used as it is.
As to coding efficiency, since a coordinate position achieving the minimum in a search range is indicated, there is an advantage that can achieve control to reduce the vector coding amount in terms of the amount of data of an output coded bit stream.
However, the quantization parameters of different color components in the coding block at the same position or therearound may vary, and the motion vector of the color component deviates from actual motion of an object, thereby there may exist a fear that influence on image quality, such as out-of-color registration, occurs.
According to the method disclosed in Japanese Patent Application Laid-Open No. 2007-133823, motion vector detection is not performed on B and R color components. Instead, the motion vector of the Y or G component is solely applied to all the color components. Accordingly, the problem, such as the out-of-color registration, cannot be solved.